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IT Horror Stories with Jack Smith · August 17 · 25 min

Microcode Mayhem : How a Mainframe Timer Bug Slowed an Entire Development Team

One day, a busy IBM-compatible mainframe began slowing to a crawl. Hundreds of programmers noticed immediately—some even resorted to banging on the server room window whenever the system became unusable. The problem appeared after a microcode upgrade, but nothing pointed to an obvious cause. In this episode, Jack Smith talks to Matt about an extraordinary piece of low-level troubleshooting that led deep into the internals of VM, interrupt handling and timer queues. Armed with nothing more than hex dumps, careful reasoning and a willingness to modify live memory, Matt uncovered a vendor bug and devised a temporary workaround that kept the system running until a permanent fix arrived. It's a fascinating story of how understanding a system at its lowest levels can solve problems that no diagnostic tool could explain. Want even more info ? Read our show notes and related blog post related to this episode : https://blog.ithorrorstories.eu/episode-20-microcode-mayhem/ 00:00 Introduction 01:07 Mainframe Tech 04:38 Microcode Issues 07:19 Timer Interrupts 15:02 Modern Comparisons 19:44 Final Insights 22:18 Conclusion

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One day, a busy IBM-compatible mainframe began slowing to a crawl. Hundreds of programmers noticed immediately—some even resorted to banging on the server room window whenever the system became unusable. The problem appeared after a microcode upgrade, but nothing pointed to an obvious cause.

In this episode, Jack Smith talks to Matt about an extraordinary piece of low-level troubleshooting that led deep into the internals of VM, interrupt handling and timer queues. Armed with nothing more than hex dumps, careful reasoning and a willingness to modify live memory, Matt uncovered a vendor bug and devised a temporary workaround that kept the system running until a permanent fix arrived. It's a fascinating story of how understanding a system at its lowest levels can solve problems that no diagnostic tool could explain.

Want even more info ? Read our show notes and related blog post related to this episode : https://blog.ithorrorstories.eu/episode-20-microcode-mayhem/

00:00 Introduction 01:07 Mainframe Tech 04:38 Microcode Issues 07:19 Timer Interrupts 15:02 Modern Comparisons 19:44 Final Insights 22:18 Conclusion

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